usiung supplimental breakers on the load side of a VDC Power Supply

I am struggling with finding anything to support this.

I really do not want to use UL489 breakers for the load side since they are expensive compared to UL1077 breakers but I cannot find anything to point me in the right direction.

Any thoughts/practices?

I am working with a 508A panel shop so this is important to get right....

I am not a UL expert and never needed to UL508 certify a design, but I had a a panel pass a CSA inspection last week using this method.

Page 2 of this PDF is how I typically use them:
https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/industrial-miniature-circuit-breakers/FAZ-product-guide-pg01101009e.pdf

I use the UL489 breakers as the primary protection of the load side of a power supply or transformer and use the UL1077 breakers for any branch circuits after that breaker, except for the listed devices.

I'm definitely interested if any one has any additional information.

I will typically use a 1) UL489 on the output of the Power Supply and then use that to feed fuses for each circuit going out of my panel. Haven't had one issue with my UL Inspector doing that.

Bullzi's method is definitely safe. I think a case could even be made that if the power supply is UL listed and is self-protecting you could use only UL1077 supp. protectors on the output side as long as all the wiring is capable of handling the rated power supply current.

I guess that is ultimately the point. You need to be able to guarantee that the wiring in a branch will not see extended current in excess of its rating. A UL489 breaker is rated to do that. A supp. protector is not.

Keith

kamenges said:

Bullzi's method is definitely safe. I think a case could even be made that if the power supply is UL listed and is self-protecting you could use only UL1077 supp. protectors on the output side as long as all the wiring is capable of handling the rated power supply current.

I guess that is ultimately the point. You need to be able to guarantee that the wiring in a branch will not see extended current in excess of its rating. A UL489 breaker is rated to do that. A supp. protector is not.

Keith

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Keith,

The part I bolded is what is getting me. It seems like if I use a 20A CB for the Feeder and 1A CB for the supplementals, I still need to use 12AWG on both sides of the 1A CB.

Am I interpreting this correctly?

Another wrinkle is that this 24VDC will also leave the cabinet to provide power for sensors and switches at each station on the line.

I think I might be stuck.....

You are interpreting my response correctly, assuming I am interpreting UL508A correctly. Only overcurrent protection rated for use as branch circuit protection can be used to define the available current in a for the purposed of conductor sizing. There are a few modifiers but this is generally true.

I think when it comes to 24VDC it doesn't matter if the conductors stay inside the enclosure or leave the enclosure relative to conductor protection.

Then again, I have heard from one UL panel shop that circuits below 30VDC are not covered by UL508A. However, based on what I can tell, these circuits would also have to be Class 2 or Class 3 circuits to be exempt.

Keith

Keith,

Thanks for your input!

Use a class 2 supply.

Then it doesn't really matter what comes after the power supply as long as the branch circuit feeding it is ok.

If you have many loads - my recommendation is to use a non-class 2 supply with something like a Murrelektronik Mico or Siemens Selectivity module. They provide class 2 circuit protection.

Most 24VDC equipment specifies a class 2 supply in the spec sheets anyway.

Rson said:

Use a class 2 supply.

Then it doesn't really matter what comes after the power supply as long as the branch circuit feeding it is ok.

If you have many loads - my recommendation is to use a non-class 2 supply with something like a Murrelektronik Mico or Siemens Selectivity module. They provide class 2 circuit protection.

Most 24VDC equipment specifies a class 2 supply in the spec sheets anyway.

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Rson,

Thanks for pointing me towards the Mico! I am very interested and am getting quotes today.

Rson said:

Use a class 2 supply.

Then it doesn't really matter what comes after the power supply as long as the branch circuit feeding it is ok.

If you have many loads - my recommendation is to use a non-class 2 supply with something like a Murrelektronik Mico or Siemens Selectivity module. They provide class 2 circuit protection.

Most 24VDC equipment specifies a class 2 supply in the spec sheets anyway.

Click to expand...

This ^^^^


You are not required to have circuit protection at all on the output of a Class 2 power supply, so it makes no difference if they are UL1077 or UL489. It's basically one of the few places you CAN use UL1077 listed devices.